The production of serine ?-lactamase is one of the primary resistance mechanisms used by Gram-negative bacterial pathogens against ?-lactam antibiotics, which include the widely used penicillins and cephalosporins, as well as last resort antibiotics such as the carbapenems. The development of novel ?-lactamase inhibitors is a pressing need underscored by the continuing mutation of ?-lactamases. We propose the development of high affinity non-covalent ?-lactamase inhibitors by targeting conserved structural motifs, particularly those essential for extended spectrum ?-lactamase activity. Prototypes of these inhibitors have already been identified. Specifically, using the CTX-M Class A ?-lactamases as a model system, we aim to: 1) apply a fragment-based and structure-guided approach to develop novel ?-lactamase inhibitor chemotypes; 2) study resistance and ligand binding by ultrahigh-resolution and room-temperature X-ray crystallography; and 3) investigate the evolution of resistance mutations against non-covalent inhibitors. These experiments will lead to new ?- lactamase inhibitors with clinical potential, while providing a deeper understanding of ?-lactamase mutations relevant to resistance evolution.